Project Summary Recent studies suggest that nitrate and nitrite molecules have profound beneficial effects to human health, though they were thought to be cancerous since 1970s. To maximize their pharmaceutical potential, we need to first understand how nitrate and nitrite circulate in humans. Such circulation depends on two critical events: active accumulation of nitrate mediated by sialin transporters in salivary glands, as well as nitrate uptake and nitrite secretion by commensal bacteria in the mouth. Thus, nitrate/nitrite molecules have to cross different cellular membranes multiple times, before being used by humans. These translocation processes are mediated by a group of membrane proteins called nitrate transporters. To understand how these transporters function, we solved high-resolution crystal structures of NarK from E. coli, and further demonstrated that, surprisingly, NarK is a nitrate/nitrite exchanger. Despite the progress we made, the detailed molecular mechanisms of nitrate/nitrite translocation are still largely unknown. In this proposal, we aim to fill the knowledge gap by: 1) understand substrate selectivity and conformational flexibility using directed-mutagenesis of NarK, so to better understand the function of NarK at the molecular level; 2) obtain high-resolution structures of NarK in previously unobserved conformation, so we can reconstruct the complete transport cycle of NarK; 3) explore the structure-function relationship of human nitrate transporter sialin, so we will understand the similarities and differences among nitrate transporters from diverse species. Overall, upon completion of the proposal, we expect to expand our general understanding of the nitrate/nitrite transport, and shed light on the crucial roles that nitrate transporters (both eukaryotic and prokaryotic) play in nitrate/nitrite circulation. The knowledge gained here will facilitate potential drug development related to bacterial nitrate transporters and human sialin.